PANIC: XX000: right sibling is not next child in "pg_depend_reference_index"

DEBUG:  00000: StartTransactionCommand
LOCATION:  start_xact_command, postgres.c:1981
DEBUG:  00000: ProcessQuery
LOCATION:  ProcessQuery, pquery.c:128
STATEMENT:  INSERT INTO links VALUES ($1, $2, $3)
PANIC:  XX000: right sibling is not next child in
"pg_depend_reference_index"
LOCATION:  _bt_pagedel, nbtpage.c:983
DEBUG:  00000: reaping dead processes
LOCATION:  reaper, postmaster.c:2027
LOG:  00000: autovacuum process (PID 5338) was terminated by signal 6
LOCATION:  LogChildExit, postmaster.c:2431
LOG:  00000: terminating any other active server processes

This error is occurring about once a day.  I believe it the result of a
less-then-graceful shutdown.  Using 8.1.0 under OS/X.  Have tried
vacuuming and reindexing to no avail